Decoupling of Mg from Sr–Nd isotopic compositions in Variscan subduction-related plutonic rocks from the Bohemian Massif: implications for mantle enrichment processes and genesis of orogenic ultrapotassic magmatic rocks

The Moldanubian Zone of the Bohemian Massif was intruded by three Variscan (c. 354–335 Ma) subduction-related plutonic suites. Their mantle sources evolved from CHUR-like (low-K calc-alkaline suite), through slightly enriched (εNd346\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\varepsilon_{Nd}^{346}$$\end{document} ~ − 3; high-K calc-alkaline suite—HKCA) to strongly enriched (εNd337\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\varepsilon_{Nd}^{337}$$\end{document} < − 7.5; (ultra-)potassic suite—UK). This evolution has been previously interpreted in terms of Andean-type subduction passing to deep subduction/relamination of the Saxothuringian continental crust, metasomatizing the mantle source of the HKCA and, more significantly, of the UK suite. This is in accord with the heterogeneity of lithospheric mantle fragments (spinel/garnet peridotites, garnet pyroxenites, eclogites, glimmerites…) sampled by the high-grade Moldanubian orogenic root. Newly acquired Mg isotopic compositions (δ26Mg = − 0.12 to − 0.53‰) vary over similarly broad intervals within each suite. Majority of the most magnesian samples fall within the range of local orogenic mantle peridotites (− 0.33 to − 0.29‰) or close to the global mantle average (− 0.25‰). This implies that the δ26Mg of the progressively metasomatized harzburgitic mantle was mostly buffered by the mantle end-member, while its mantle-incompatible elemental/related isotopic (Sr–Nd–Pb) signal was swamped by the crustally derived contribution. The crustal contaminant, source of the metasomatic fluids, had to be dominated by Mg-poor, felsic metaigneous >  > clastic metasedimentary material. Subducted carbonates were of limited importance, as recorded by lowered δ26Mg values in several mafic UK samples and in a glimmerite vein cutting the peridotites (− 0.44‰). The Mg isotopic variation in less magnesian potassic rock types reflects a complex interplay between source heterogeneity, equilibrium fractionation during fractional crystallization, kinetic fractionation due to chemical diffusion during magma mixing and/or AFC-style contamination by carbonate-derived fluids.